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The pickle module implements binary protocols for serializing and
de-serializing a Python object structure. “Pickling” is the process
whereby a Python object hierarchy is converted into a byte stream, and
“unpickling” is the inverse operation, whereby a byte stream
(from a binary file or bytes-like object) is converted
back into an object hierarchy. Pickling (and unpickling) is alternatively
known as “serialization”, “marshalling,” [1] or “flattening”; however, to
avoid confusion, the terms used here are “pickling” and “unpickling”.

Warning

The pickle module is not secure against erroneous or maliciously
constructed data. Never unpickle data received from an untrusted or
unauthenticated source.

Python has a more primitive serialization module called marshal, but in
general pickle should always be the preferred way to serialize Python
objects. marshal exists primarily to support Python’s .pyc
files.

The pickle module keeps track of the objects it has already serialized,
so that later references to the same object won’t be serialized again.
marshal doesn’t do this.

This has implications both for recursive objects and object sharing. Recursive
objects are objects that contain references to themselves. These are not
handled by marshal, and in fact, attempting to marshal recursive objects will
crash your Python interpreter. Object sharing happens when there are multiple
references to the same object in different places in the object hierarchy being
serialized. pickle stores such objects only once, and ensures that all
other references point to the master copy. Shared objects remain shared, which
can be very important for mutable objects.

marshal cannot be used to serialize user-defined classes and their
instances. pickle can save and restore class instances transparently,
however the class definition must be importable and live in the same module as
when the object was stored.

The marshal serialization format is not guaranteed to be portable
across Python versions. Because its primary job in life is to support
.pyc files, the Python implementers reserve the right to change the
serialization format in non-backwards compatible ways should the need arise.
The pickle serialization format is guaranteed to be backwards compatible
across Python releases provided a compatible pickle protocol is chosen and
pickling and unpickling code deals with Python 2 to Python 3 type differences
if your data is crossing that unique breaking change language boundary.

JSON is a text serialization format (it outputs unicode text, although
most of the time it is then encoded to utf-8), while pickle is
a binary serialization format;

JSON is human-readable, while pickle is not;

JSON is interoperable and widely used outside of the Python ecosystem,
while pickle is Python-specific;

JSON, by default, can only represent a subset of the Python built-in
types, and no custom classes; pickle can represent an extremely large
number of Python types (many of them automatically, by clever usage
of Python’s introspection facilities; complex cases can be tackled by
implementing specific object APIs).

The data format used by pickle is Python-specific. This has the
advantage that there are no restrictions imposed by external standards such as
JSON or XDR (which can’t represent pointer sharing); however it means that
non-Python programs may not be able to reconstruct pickled Python objects.

There are currently 5 different protocols which can be used for pickling.
The higher the protocol used, the more recent the version of Python needed
to read the pickle produced.

Protocol version 0 is the original “human-readable” protocol and is
backwards compatible with earlier versions of Python.

Protocol version 1 is an old binary format which is also compatible with
earlier versions of Python.

Protocol version 2 was introduced in Python 2.3. It provides much more
efficient pickling of new-style classes. Refer to PEP 307 for
information about improvements brought by protocol 2.

Protocol version 3 was added in Python 3.0. It has explicit support for
bytes objects and cannot be unpickled by Python 2.x. This was
the default protocol in Python 3.0–3.7.

Protocol version 4 was added in Python 3.4. It adds support for very large
objects, pickling more kinds of objects, and some data format
optimizations. It is the default protocol starting with Python 3.8.
Refer to PEP 3154 for information about improvements brought by
protocol 4.

Note

Serialization is a more primitive notion than persistence; although
pickle reads and writes file objects, it does not handle the issue of
naming persistent objects, nor the (even more complicated) issue of concurrent
access to persistent objects. The pickle module can transform a complex
object into a byte stream and it can transform the byte stream into an object
with the same internal structure. Perhaps the most obvious thing to do with
these byte streams is to write them onto a file, but it is also conceivable to
send them across a network or store them in a database. The shelve
module provides a simple interface to pickle and unpickle objects on
DBM-style database files.

To serialize an object hierarchy, you simply call the dumps() function.
Similarly, to de-serialize a data stream, you call the loads() function.
However, if you want more control over serialization and de-serialization,
you can create a Pickler or an Unpickler object, respectively.

An integer, the default protocol version used
for pickling. May be less than HIGHEST_PROTOCOL. Currently the
default protocol is 4, first introduced in Python 3.4 and incompatible
with previous versions.

Changed in version 3.0: The default protocol is 3.

Changed in version 3.8: The default protocol is 4.

The pickle module provides the following functions to make the pickling
process more convenient:

Write a pickled representation of obj to the open file objectfile.
This is equivalent to Pickler(file,protocol).dump(obj).

The optional protocol argument, an integer, tells the pickler to use
the given protocol; supported protocols are 0 to HIGHEST_PROTOCOL.
If not specified, the default is DEFAULT_PROTOCOL. If a negative
number is specified, HIGHEST_PROTOCOL is selected.

The file argument must have a write() method that accepts a single bytes
argument. It can thus be an on-disk file opened for binary writing, an
io.BytesIO instance, or any other custom object that meets this
interface.

If fix_imports is true and protocol is less than 3, pickle will try to
map the new Python 3 names to the old module names used in Python 2, so
that the pickle data stream is readable with Python 2.

Read a pickled object representation from the open file objectfile and return the reconstituted object hierarchy specified therein.
This is equivalent to Unpickler(file).load().

The protocol version of the pickle is detected automatically, so no
protocol argument is needed. Bytes past the pickled object’s
representation are ignored.

The argument file must have two methods, a read() method that takes an
integer argument, and a readline() method that requires no arguments. Both
methods should return bytes. Thus file can be an on-disk file opened for
binary reading, an io.BytesIO object, or any other custom object
that meets this interface.

Optional keyword arguments are fix_imports, encoding and errors,
which are used to control compatibility support for pickle stream generated
by Python 2. If fix_imports is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The encoding and
errors tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The encoding can
be ‘bytes’ to read these 8-bit string instances as bytes objects.
Using encoding='latin1' is required for unpickling NumPy arrays and
instances of datetime, date and
time pickled by Python 2.

The protocol version of the pickle is detected automatically, so no
protocol argument is needed. Bytes past the pickled object’s
representation are ignored.

Optional keyword arguments are fix_imports, encoding and errors,
which are used to control compatibility support for pickle stream generated
by Python 2. If fix_imports is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The encoding and
errors tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The encoding can
be ‘bytes’ to read these 8-bit string instances as bytes objects.
Using encoding='latin1' is required for unpickling NumPy arrays and
instances of datetime, date and
time pickled by Python 2.

The optional protocol argument, an integer, tells the pickler to use
the given protocol; supported protocols are 0 to HIGHEST_PROTOCOL.
If not specified, the default is DEFAULT_PROTOCOL. If a negative
number is specified, HIGHEST_PROTOCOL is selected.

The file argument must have a write() method that accepts a single bytes
argument. It can thus be an on-disk file opened for binary writing, an
io.BytesIO instance, or any other custom object that meets this
interface.

If fix_imports is true and protocol is less than 3, pickle will try to
map the new Python 3 names to the old module names used in Python 2, so
that the pickle data stream is readable with Python 2.

A pickler object’s dispatch table is a registry of reduction
functions of the kind which can be declared using
copyreg.pickle(). It is a mapping whose keys are classes
and whose values are reduction functions. A reduction function
takes a single argument of the associated class and should
conform to the same interface as a __reduce__()
method.

By default, a pickler object will not have a
dispatch_table attribute, and it will instead use the
global dispatch table managed by the copyreg module.
However, to customize the pickling for a specific pickler object
one can set the dispatch_table attribute to a dict-like
object. Alternatively, if a subclass of Pickler has a
dispatch_table attribute then this will be used as the
default dispatch table for instances of that class.

Special reducer that can be defined in Pickler subclasses. This
method has priority over any reducer in the dispatch_table. It
should conform to the same interface as a __reduce__() method, and
can optionally return NotImplemented to fallback on
dispatch_table-registered reducers to pickle obj.

Deprecated. Enable fast mode if set to a true value. The fast mode
disables the usage of memo, therefore speeding the pickling process by not
generating superfluous PUT opcodes. It should not be used with
self-referential objects, doing otherwise will cause Pickler to
recurse infinitely.

The protocol version of the pickle is detected automatically, so no
protocol argument is needed.

The argument file must have two methods, a read() method that takes an
integer argument, and a readline() method that requires no arguments. Both
methods should return bytes. Thus file can be an on-disk file object
opened for binary reading, an io.BytesIO object, or any other
custom object that meets this interface.

Optional keyword arguments are fix_imports, encoding and errors,
which are used to control compatibility support for pickle stream generated
by Python 2. If fix_imports is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The encoding and
errors tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The encoding can
be ‘bytes’ to read these 8-bit string instances as bytes objects.

Read a pickled object representation from the open file object given in
the constructor, and return the reconstituted object hierarchy specified
therein. Bytes past the pickled object’s representation are ignored.

Import module if necessary and return the object called name from it,
where the module and name arguments are str objects. Note,
unlike its name suggests, find_class() is also used for finding
functions.

Subclasses may override this to gain control over what type of objects and
how they can be loaded, potentially reducing security risks. Refer to
Restricting Globals for details.

Attempts to pickle unpicklable objects will raise the PicklingError
exception; when this happens, an unspecified number of bytes may have already
been written to the underlying file. Trying to pickle a highly recursive data
structure may exceed the maximum recursion depth, a RecursionError will be
raised in this case. You can carefully raise this limit with
sys.setrecursionlimit().

Note that functions (built-in and user-defined) are pickled by “fully qualified”
name reference, not by value. [2] This means that only the function name is
pickled, along with the name of the module the function is defined in. Neither
the function’s code, nor any of its function attributes are pickled. Thus the
defining module must be importable in the unpickling environment, and the module
must contain the named object, otherwise an exception will be raised. [3]

Similarly, classes are pickled by named reference, so the same restrictions in
the unpickling environment apply. Note that none of the class’s code or data is
pickled, so in the following example the class attribute attr is not
restored in the unpickling environment:

classFoo:attr='A class attribute'picklestring=pickle.dumps(Foo)

These restrictions are why picklable functions and classes must be defined in
the top level of a module.

Similarly, when class instances are pickled, their class’s code and data are not
pickled along with them. Only the instance data are pickled. This is done on
purpose, so you can fix bugs in a class or add methods to the class and still
load objects that were created with an earlier version of the class. If you
plan to have long-lived objects that will see many versions of a class, it may
be worthwhile to put a version number in the objects so that suitable
conversions can be made by the class’s __setstate__() method.

In this section, we describe the general mechanisms available to you to define,
customize, and control how class instances are pickled and unpickled.

In most cases, no additional code is needed to make instances picklable. By
default, pickle will retrieve the class and the attributes of an instance via
introspection. When a class instance is unpickled, its __init__() method
is usually not invoked. The default behaviour first creates an uninitialized
instance and then restores the saved attributes. The following code shows an
implementation of this behaviour:

In protocols 2 and newer, classes that implements the
__getnewargs_ex__() method can dictate the values passed to the
__new__() method upon unpickling. The method must return a pair
(args,kwargs) where args is a tuple of positional arguments
and kwargs a dictionary of named arguments for constructing the
object. Those will be passed to the __new__() method upon
unpickling.

You should implement this method if the __new__() method of your
class requires keyword-only arguments. Otherwise, it is recommended for
compatibility to implement __getnewargs__().

This method serves a similar purpose as __getnewargs_ex__(), but
supports only positional arguments. It must return a tuple of arguments
args which will be passed to the __new__() method upon unpickling.

Classes can further influence how their instances are pickled; if the class
defines the method __getstate__(), it is called and the returned object
is pickled as the contents for the instance, instead of the contents of the
instance’s dictionary. If the __getstate__() method is absent, the
instance’s __dict__ is pickled as usual.

Upon unpickling, if the class defines __setstate__(), it is called with
the unpickled state. In that case, there is no requirement for the state
object to be a dictionary. Otherwise, the pickled state must be a dictionary
and its items are assigned to the new instance’s dictionary.

As we shall see, pickle does not use directly the methods described above. In
fact, these methods are part of the copy protocol which implements the
__reduce__() special method. The copy protocol provides a unified
interface for retrieving the data necessary for pickling and copying
objects. [4]

The interface is currently defined as follows. The __reduce__() method
takes no argument and shall return either a string or preferably a tuple (the
returned object is often referred to as the “reduce value”).

If a string is returned, the string should be interpreted as the name of a
global variable. It should be the object’s local name relative to its
module; the pickle module searches the module namespace to determine the
object’s module. This behaviour is typically useful for singletons.

When a tuple is returned, it must be between two and six items long.
Optional items can either be omitted, or None can be provided as their
value. The semantics of each item are in order:

A callable object that will be called to create the initial version of the
object.

A tuple of arguments for the callable object. An empty tuple must be given
if the callable does not accept any argument.

Optionally, the object’s state, which will be passed to the object’s
__setstate__() method as previously described. If the object has no
such method then, the value must be a dictionary and it will be added to
the object’s __dict__ attribute.

Optionally, an iterator (and not a sequence) yielding successive items.
These items will be appended to the object either using
obj.append(item) or, in batch, using obj.extend(list_of_items).
This is primarily used for list subclasses, but may be used by other
classes as long as they have append() and extend() methods with
the appropriate signature. (Whether append() or extend() is
used depends on which pickle protocol version is used as well as the number
of items to append, so both must be supported.)

Optionally, an iterator (not a sequence) yielding successive key-value
pairs. These items will be stored to the object using obj[key]=value. This is primarily used for dictionary subclasses, but may be used
by other classes as long as they implement __setitem__().

Optionally, a callable with a (obj,state) signature. This
callable allows the user to programmatically control the state-updating
behavior of a specific object, instead of using obj’s static
__setstate__() method. If not None, this callable will have
priority over obj’s __setstate__().

New in version 3.8: The optional sixth tuple item, (obj,state), was added.

Alternatively, a __reduce_ex__() method may be defined. The only
difference is this method should take a single integer argument, the protocol
version. When defined, pickle will prefer it over the __reduce__()
method. In addition, __reduce__() automatically becomes a synonym for
the extended version. The main use for this method is to provide
backwards-compatible reduce values for older Python releases.

For the benefit of object persistence, the pickle module supports the
notion of a reference to an object outside the pickled data stream. Such
objects are referenced by a persistent ID, which should be either a string of
alphanumeric characters (for protocol 0) [5] or just an arbitrary object (for
any newer protocol).

The resolution of such persistent IDs is not defined by the pickle
module; it will delegate this resolution to the user defined methods on the
pickler and unpickler, persistent_id() and
persistent_load() respectively.

To pickle objects that have an external persistent id, the pickler must have a
custom persistent_id() method that takes an object as an
argument and returns either None or the persistent id for that object.
When None is returned, the pickler simply pickles the object as normal.
When a persistent ID string is returned, the pickler will pickle that object,
along with a marker so that the unpickler will recognize it as a persistent ID.

To unpickle external objects, the unpickler must have a custom
persistent_load() method that takes a persistent ID object and
returns the referenced object.

Here is a comprehensive example presenting how persistent ID can be used to
pickle external objects by reference.

# Simple example presenting how persistent ID can be used to pickle# external objects by reference.importpickleimportsqlite3fromcollectionsimportnamedtuple# Simple class representing a record in our database.MemoRecord=namedtuple("MemoRecord","key, task")classDBPickler(pickle.Pickler):defpersistent_id(self,obj):# Instead of pickling MemoRecord as a regular class instance, we emit a# persistent ID.ifisinstance(obj,MemoRecord):# Here, our persistent ID is simply a tuple, containing a tag and a# key, which refers to a specific record in the database.return("MemoRecord",obj.key)else:# If obj does not have a persistent ID, return None. This means obj# needs to be pickled as usual.returnNoneclassDBUnpickler(pickle.Unpickler):def__init__(self,file,connection):super().__init__(file)self.connection=connectiondefpersistent_load(self,pid):# This method is invoked whenever a persistent ID is encountered.# Here, pid is the tuple returned by DBPickler.cursor=self.connection.cursor()type_tag,key_id=pidiftype_tag=="MemoRecord":# Fetch the referenced record from the database and return it.cursor.execute("SELECT * FROM memos WHERE key=?",(str(key_id),))key,task=cursor.fetchone()returnMemoRecord(key,task)else:# Always raises an error if you cannot return the correct object.# Otherwise, the unpickler will think None is the object referenced# by the persistent ID.raisepickle.UnpicklingError("unsupported persistent object")defmain():importioimportpprint# Initialize and populate our database.conn=sqlite3.connect(":memory:")cursor=conn.cursor()cursor.execute("CREATE TABLE memos(key INTEGER PRIMARY KEY, task TEXT)")tasks=('give food to fish','prepare group meeting','fight with a zebra',)fortaskintasks:cursor.execute("INSERT INTO memos VALUES(NULL, ?)",(task,))# Fetch the records to be pickled.cursor.execute("SELECT * FROM memos")memos=[MemoRecord(key,task)forkey,taskincursor]# Save the records using our custom DBPickler.file=io.BytesIO()DBPickler(file).dump(memos)print("Pickled records:")pprint.pprint(memos)# Update a record, just for good measure.cursor.execute("UPDATE memos SET task='learn italian' WHERE key=1")# Load the records from the pickle data stream.file.seek(0)memos=DBUnpickler(file,conn).load()print("Unpickled records:")pprint.pprint(memos)if__name__=='__main__':main()

Here’s an example that shows how to modify pickling behavior for a class.
The TextReader class opens a text file, and returns the line number and
line contents each time its readline() method is called. If a
TextReader instance is pickled, all attributes except the file object
member are saved. When the instance is unpickled, the file is reopened, and
reading resumes from the last location. The __setstate__() and
__getstate__() methods are used to implement this behavior.

classTextReader:"""Print and number lines in a text file."""def__init__(self,filename):self.filename=filenameself.file=open(filename)self.lineno=0defreadline(self):self.lineno+=1line=self.file.readline()ifnotline:returnNoneifline.endswith('\n'):line=line[:-1]return"%i: %s"%(self.lineno,line)def__getstate__(self):# Copy the object's state from self.__dict__ which contains# all our instance attributes. Always use the dict.copy()# method to avoid modifying the original state.state=self.__dict__.copy()# Remove the unpicklable entries.delstate['file']returnstatedef__setstate__(self,state):# Restore instance attributes (i.e., filename and lineno).self.__dict__.update(state)# Restore the previously opened file's state. To do so, we need to# reopen it and read from it until the line count is restored.file=open(self.filename)for_inrange(self.lineno):file.readline()# Finally, save the file.self.file=file

Sometimes, dispatch_table may not be flexible enough.
In particular we may want to customize pickling based on another criterion
than the object’s type, or we may want to customize the pickling of
functions and classes.

For those cases, it is possible to subclass from the Pickler class and
implement a reducer_override() method. This method can return an
arbitrary reduction tuple (see __reduce__()). It can alternatively return
NotImplemented to fallback to the traditional behavior.

Here is a simple example where we allow pickling and reconstructing
a given class:

importioimportpickleclassMyClass:my_attribute=1classMyPickler(pickle.Pickler):defreducer_override(self,obj):"""Custom reducer for MyClass."""ifgetattr(obj,"__name__",None)=="MyClass":returntype,(obj.__name__,obj.__bases__,{'my_attribute':obj.my_attribute})else:# For any other object, fallback to usual reductionreturnNotImplementedf=io.BytesIO()p=MyPickler(f)p.dump(MyClass)delMyClassunpickled_class=pickle.loads(f.getvalue())assertisinstance(unpickled_class,type)assertunpickled_class.__name__=="MyClass"assertunpickled_class.my_attribute==1

By default, unpickling will import any class or function that it finds in the
pickle data. For many applications, this behaviour is unacceptable as it
permits the unpickler to import and invoke arbitrary code. Just consider what
this hand-crafted pickle data stream does when loaded:

In this example, the unpickler imports the os.system() function and then
apply the string argument “echo hello world”. Although this example is
inoffensive, it is not difficult to imagine one that could damage your system.

For this reason, you may want to control what gets unpickled by customizing
Unpickler.find_class(). Unlike its name suggests,
Unpickler.find_class() is called whenever a global (i.e., a class or
a function) is requested. Thus it is possible to either completely forbid
globals or restrict them to a safe subset.

Here is an example of an unpickler allowing only few safe classes from the
builtins module to be loaded:

As our examples shows, you have to be careful with what you allow to be
unpickled. Therefore if security is a concern, you may want to consider
alternatives such as the marshalling API in xmlrpc.client or
third-party solutions.

Recent versions of the pickle protocol (from protocol 2 and upwards) feature
efficient binary encodings for several common features and built-in types.
Also, the pickle module has a transparent optimizer written in C.

The limitation on alphanumeric characters is due to the fact
the persistent IDs, in protocol 0, are delimited by the newline
character. Therefore if any kind of newline characters occurs in
persistent IDs, the resulting pickle will become unreadable.